1. Field of the Invention
The present invention generally relates to heated die plates for underwater pelletizers and, more specifically, to center heated die plates for underwater pelletizers.
2. Description of the Related Art
Underwater pelletizers are well known and include a die plate having a plurality of extrusion orifices extending from an upstream side of the die plate to a downstream side for extruding strands of molten polymer or other extrudable materials. The extrusion orifices are arranged in one or more circular patterns concentrically of and in radially spaced relation to the center of the die plate and in spaced relation to each other. The extruded strands of polymer or other material are cut into pellets by rotating cutter blades in a water box. The blades cut the strands into pellets which have been cooled and solidified by the water in the water box. A slurry of pellets and water is discharged from the water box for subsequent removal of moisture from the exterior surface of the pellets.
It is desirable to maintain the die plate and orifices at an elevated temperature for optimum extrusion of the molten polymer through the extrusion orifices. Various efforts have been made to heat the die plate including the use of passageways in the die plate through which a heating fluid is circulated. Also, radial cavities have been formed in the periphery of die plates into which electric cartridge heaters are inserted from the external periphery of the die plate. Such known structures are typically illustrated in the following U.S. patents owned by the assignee of this application and include disclosures of heated die plates: U.S. Pat. Nos. 4,123,207; 4,621,996; 4,728,276; 5,403,176; and 5,624,688.
As illustrated in the above patents, heated die plates with concentric circular patterns of extrusion orifices are generally well known as well as passageways for heated fluid and cavities or pockets for receiving cartridge heaters. Such die plates require that portions of the die plates through which extrusion orifices would normally extend be occupied by the passageways for heating fluid or cavities for cartridge heaters. The passageways or cavities thus reduce the number of extrusion orifices that can be incorporated into the die plate since the passageways carrying the heating fluid or the cavities or pockets receiving the cartridge heaters require that the extrusion orifices be omitted from a portion of the area of the die plate. Also, the heated fluid passageways and heat cartridge cavities tend to concentrate heat radiation in areas closely adjacent to the passageways or cavities.